1. Field of the Invention
The present invention relates to a tilt sensor (tiltmeter) in which the position of an air bubble enclosed in a sealed container is measured to detect a tilt angle of the sealed container, and a mask and a coat holder, which are adapted to form two electrodes on an upper plate which constitutes the sealed container.
2. Description of the Related Art
In general, a tilt sensor is provided in survey instruments, aircraft, or automobiles, etc., to measure a tilt angle thereof, wherein the position of an air bubble contained in a liquid enclosed in a sealed container is electrically detected.
The assignee of the present application has proposed a tilt sensor in which the sealed container is comprised of a top cover plate (an upper end plate) which is provided on one surface thereof with a recess whose bottom surface is made of a toroidal surface (i.e., a curved surface having different curvatures in two orthogonal directions at the center thereof), a cylindrical body, and a bottom plate to relax the mounting precision of the tilt sensor in a direction perpendicular to a direction (tilt angle detection direction) in which the tilt angle is detected (Japanese Patent Application No. 8-105834) and counterpart U.S. application Ser. No. 08/840,074, filed Apr. 24, 1997, entitled "Tilt Sensor".
In the tilt sensor proposed in JPA '834, the toroidal surface of the top cover plate is formed such that the curvature in the tilt angle detection direction is smaller than the curvature in the direction perpendicular thereto. A pair of upper electrodes made of a platinum coating are provided on the toroidal surface in a line-symmetry arrangement with respect to the center line which extends in the direction perpendicular to the tilt angle detection direction.
The top cover plate constitutes the sealed container in which the electrolyte containing an air bubble, together with the bottom plate on which the upper electrodes are formed and the cylindrical body. The sealed container is immovably hung from a holder which forms a housing of the tilt sensor. The sealed container is positioned so that the tilt angle detection direction of the top cover plate is identical to the inclination angle detection angle of the tilt sensor. The holder is closed by a cover to complete the tilt sensor.
In the proposed tilt sensor, if a side surface of an object to which the tilt sensor is to be attached to detect the tilt angle does not lie in the vertical plane, the air bubble is moved away from the inner wall of the cylindrical body and hence the precise measurement of the tilt angle can be carried out.
The upper electrodes provided on the toroidal surface are made of a platinum thin layer deposited thereon by high-frequency sputtering, using a predetermined shape of mask on the toroidal surface. The portions of the mask corresponding to the upper electrodes must be juxtaposed in the tilt angle detection direction of the top cover plate. Otherwise, the curvature of the toroidal surface in the direction in which the upper electrodes formed using the mask are juxtaposed is not identical to the value on design, thus resulting in a variation in the displacement of the air bubble depending on the inclination of the sealed container. Thus, the tilt angle detection sensitivity (output/tilt angle) becomes irregular.
The positioning operation to precisely determine the position of the mask relative to the top cover plate so that the portions of the mask that correspond to the upper electrodes are juxtaposed in the tilt angle detection direction is cumbersome. Moreover, even if the mask is precisely positioned relative to the top cover plate, there is a possibility that the upper electrodes fail to be formed at correct positions, due to accidental movement of the mask during the formation of the electrodes.
In addition to the foregoing, if the upper electrodes are formed on the toroidal surface of the top cover plate at predetermined positions, a failure to connect the sealed container including the top cover plate to the holder in place results in a deviation of the curvature of the toroidal surface in the tilt angle detection direction of the tilt sensor from the design value or results in a variation in the contact surface area of the air bubble with the upper electrodes when the inclination of the tilt sensor occurs in the direction perpendicular to the tilt angle detection direction. However, the positioning operation of the sealed container which must rely upon the measurement with the operator's eyes is considerably troublesome, thus resulting in a low productivity of the tilt sensors.